The invention relates to a duplexer which is well adapted for a mobile telephone.
In duplex operation of a radio transmitter and a receiver connected to a common antenna, a duplexer is required so as to effectively isolate the transmitter and the receiver to permit simultaneous operation, especially, where the transmitting and receiving frequencies are closely spaced. A conventional duplexer is described in detail in U.S. Pat. No. 3,656,162 and Japanese Laid-open Patent Publication Nos. 87-136104 and 87-136105.
A duplexer described in U.S. Pat. No. 3,656,162 employs two coaxial cables which are respectively connected between an antenna terminal and a receiver terminal through a filter, and between a transmitter terminal through a filter and the antenna terminal. The length of one of the coaxial cables connected to the transmitter terminal, must be equal to around a quarter wavelength of the receiver frequency in order to reject the transmitter noise or harmonics at or near the receiver frequencies. The length of the other coaxial cable connected to the receiver terminal similarly must be equal to around a quarter wavelength of the transmitter frequency in order to reject the receiver noise or hermonics at or near the transmitter frequencies.
Another conventional duplexer, which has been proposed in order to reduce the size of the duplexer, is disclosed in Japanese Laid-open Patent Publication Nos. 87-136104 and 87-136105, both of which were issued on June 19, 1987 and assigned to the assignee of the present invention. The duplexer employs two distributed constant lines such as a strip line, in place of the two coaxial cables, both of which are formed on a dielectric material plate made of alumina or glass-epoxy resin materials by a thick-layer printing method or a plating method. The duplexer further employs a transmitter dielectric filter which is connected in series relation with one of the distributed constant lines to an antenna terminal and a receiver dielectric filter which is connected in series relation with the other distributed constant line to the antenna terminal. The lengths of the distributed constant line can respectively be calculated, for example, in the following manner:
An element S.sub.11 (1) of the dispersion matrix S showing a characteristic of the network comprised of the transmitter distributed constant line and the transmitter filter connected in series, is generally expressed by the following equation (1): ##EQU1## wherein r is the real part of the input impedance of the transmitter filter when the input impedence is expressed as r+j z;
z is the constant term of the imaginary part thereof: PA0 j is the imaginary unit thereof; PA0 .lambda. is the wavelength of the receiver frequency; PA0 l is the length of the transmitter distributed constant line; .theta.=.beta.l; and .beta.=2.pi./.lambda..
If the above equation (1) satisfies the following equation (2) EQU cos .theta.=z sin .theta. (2)
an input impedance of the network can be increased at and near the receiver frequencies so as to effectively reject the transmitter noise and harmonics at and near the receiver frequencies. If, in accordance with the equation (2), z sin .theta. is substituted for cos .theta. in the equation (1), equation (1) becomes ##EQU2## The element S.sub.11 (1) can also be expressed in terms of the input impedance Zin of the network as in the following equation (4) when the driving impedance of the network is assumed to be 1 ohm: EQU S.sub.11 (1)=(Zin-1)/(Zin+1), (4)
From the foregoing equations (3) and (4), the input impedance (Zin) can be expressed by the following equation (5): EQU Zin=[(1+z.sup.2)/r]-j z. (5)
According to equation (5), the real part r of the transmitter filter's input impedance can be assumed to be much less than 1 because Zin&gt;&gt;1, that is, the input impedance (Zin) of the network is required to be increased at and near the receiver frequencies, and the phase angle of the network can also be assumed as zero degree because the real part [(1+z.sup.2)/r] of the input impedance (Zin) is increased at and near the receiver frequencies. Tests have been performed with transmitter and receiver distributed constant lines (in the form of strip lines) used in mobile telephones whose transmitter frequency band is from 825 to 845 MHz and whose receiver frequency band is from 870 to 890 MHz. Each line had a width of 1.8 mm and a characteristic impedance of 50 ohm, and was formed on a glass-epoxy plate whose thickness was 1.0 mm and specific inductive capacity (dielectric constant) was 4.8. The lines were respectively connected to the transmitter and the receiver dielectric filters and were respectively set to have a phase angle around zero degree. To obtain the required large input impedance, the tests showed that the length of the transmitter line must be 84 mm and that of the receiver line must be 60 mm. However, the distributed constant lines of the above mentioned duplexer are still subject to having lengths equal to the corresponding quarter wavelengths of the rejected frequencies (these quarter wavelengths being reduced in the strip lines on the dielectric plate as compared to coaxial cables) so that formulation of a duplexer in this manner thereby presents an obstacle to further reduction of the size of the duplexer.